Thermal compression plant with heat recovery for vacuum dryers and dryer incorporating said plant

ABSTRACT

A thermal compression plant with heat recovery for vacuum dryers of industrial leathers and similar products with one or more drying chambers (4) each one composed of a heated table (2), on which the products to be dried are laid, and a hermetic cover (3). A circulating fluid in a first closed circuit (6, 7) heats each table and each drying chamber (4) is connected to a second open circuit (14) for the suction and the condensation of the steam generated by the products. The second steam circuit (14) includes at least one vacuum pump (16) of relatively low capacity placed downstream from a condenser and compression means made up by at least one blower (19) with a higher capacity of the one of the vacuum pump (16) able to compress and superheat the steam to a higher temperature to that of the heating fluid of the working tables at continuous running. The condenser (21) uses for refrigeration the heating fluid circulating in the first circuit. The advantage is the recovery of most of the energy supplied to the compression means.

DESCRIPTION

The present invention relates to a thermal compression plant with heatrecovery for vacuum dryers as well as a drier incorporating the sameplant.

With the words "thermal compression plant" it is meant a plant for thetreatment of steams coming from products that have to be dried,preferably but not exclusively industrial leathers or similar productsthat must be treated at temperatures lower than that of evaporation atatmospheric pressure.

Vacuum dryers of the above mentioned type are known, for example, asdescribed and claimed in the International patent application No.WO94/21828 and in the U.S. Pat. Nos. 3,027,651, and 3,303,574, all ofwhich are incorporated herein by reference, having characteristics onwhich the initial part of the claim 1 is based.

In these known dryers at least one heated working table is provided,hermetically covered by a removable cover to define a drying chamber forproducts. The working table is heated by a first circuit for thecirculation of a heating fluid, and the evaporation chamber is connectedto a second circuit for the suction and condensation of the steamextracted from the products at a lower temperature than that ofevaporation at atmospheric pressure.

The steam created in the drying chamber is conveyed in the workingtables' side manifolds, in which are inserted tube nests condenserscovered by a refrigerating fluid. To increase productivity, a highvacuum is provided in the evaporation chambers by means of vacuum pumpsand/or blowers placed in series.

To maintain the condensation temperature at equilibrium, the workingtables' condensers must be traversed by a refrigerating means at lowtemperature, for example chilled water which must be generated byspecific chillers and kept in suitable tanks.

This part of the plant has initially a very high cost, it is not easilymanageable, it has added maintenance costs and its energy consumptioncan be treble the consumption of the drier.

From French patent application No. 2,391,438 it is known a thermal plantfor drying under vacuum products such as wood plates, which plantcomprises a drying chamber, a first circuit for heating the dryingchamber, a second circuit for suction and condensation of the steamgenerated by the products in the drying chamber. The first circuitincludes a vacuum pump connected to a heat exchanger which has aschilling fluid the heating fluid circulating in the first circuit inorder to recover the condensation heat and part of the energy furnishedto the vacuum pump. The heat exchanger is in fluid communication withthe outside and therefore has a working pressure equal to or higher thanthe atmospheric pressure, thereby affording very low condensationefficiency. Moreover, the vacuum pump operates on a mixture of water andoil and thus entails a very low suction efficiency, thereby maintainingin the drying chamber relatively high pressure and equilibriumtemperature.

The main task of this invention is to eliminate the above mentionedinconveniences by making a thermal compression plant for vacuum dryersof industrial leathers and similar products which allows recovery ofmost of the energy for the functioning of the plant, which is highlyreliable and of simple manufacture and which has a low initial cost aswell as low management and maintenance.

A particular task is to eliminate the costly and complex condensersassociated to the tables, as well as the very expensive chilling andaccumulating plant of chilled water and moreover to recover most of themechanical work carried out on the steam although maintaining theworking temperatures relatively low.

These and other tasks, which will result more clearly further on, areachieved by a thermal compression plant with heat recovery in accordancewith claim 1, which is characterised by the fact that the second steamcircuit includes compression means which compress and superheat thesteam to a higher temperature than the temperature of the workingtables's heating fluid at continuous running, and at least a condenserhaving as chilling means the heating fluid circulating in the firstcircuit in order to recover at least part of the energy supplied to thecompression means.

The above described plant has the advantage of the simple manufacture ofthe plant and the elimination of costly circuit components.

The eventual elevation of the evaporation temperature on the products,from a strictly thermodynamic point of view, has the advantage of movingin a higher zone of the Mollier's diagram, to which corresponds lowerenergy consumption.

By opportunely choosing the compression value of the blower, acondensation temperature higher than the temperature of the evaporationon the working table is obtained; of a value included for example,between 15 and 25° C.

It will be therefore possible to condense the steam superheated incompression with the same heating water of the table generating theevaporation, recovering, apart from losses in the piping, most of thecondensation heat.

Once the plant is brought to continuous running, the only mechanicalenergy supplied is by the compression means.

The overall cost of the plant is therefore considerably reduced becausethe only costly parts are the compression means, particularly theblower. On the other hand, the remarkable costs of the chilling machine,its plant and of the accumulation tanks of the chilled water areeliminated as well as the costs of the condensers placed in each workingtable, typical of the past.

Thanks to the simple manufacturing and to the plant reliability,maintenance is also not necessary apart from the checking of the oillevel in the blower sump.

The invention will be now described in detail with reference to amanufacturing form, preferable but not exclusive, of a plant and of adryer according to the invention, illustrated as an example and notlimiting with the help of the accompaning FIG. 1. A vacuum dryer forindustrial leathers and similar products, indicated globally with thereference number 1, is made up by a plurality of working tables 2 of aknown type, of which only one is represented for simplicity, having eachone of them a upper heating surface on which leathers or other productsto be dried are laid which, for delicacy reasons must be treated atlower temperatures of that one of evaporation at atmospheric pressure.

A removable cover 3 upwardly closes the table 2, having peripheral sealsto define with the lower table a hermetically closed drying chamber 4.

Inside each table there is a coil 5 traversed by a heating fluid,normally water with opportune additives, which circulates in a firstclosed circuit having a feeding line 6 and a return line 7 and whichgives its heat to the product to be dried.

The temperature of the heating fluid is brought to a continuous runningtemperature, included, for example, between 30° C. and 80° C. accordingto the products to be dried and preferably near to approx. 50° C.

On the side of each table 2, collection chambers or manifolds 8, 9 areobtained for the steam which is released from the leathers or from theproducts to be dried and which accumulates in chamber 4. The manifolds8, 9 are connected by respective piping 10, 11 and intercepting valves12, 13 to a second open circuit at a single line 14 having an open end15 at which there is a vacuum pump 16 for the extraction of the steams.

It is observed that the vacuum pump 16 has extremely reduced the onlyfunction of extraction of the incondensible gases and therefore hasdimensions and capacity.

Preferably, between the piping 10, 11 and the line 14 a separator 17 forsolid bodies is inserted, which can have a manual or automatic discharge18.

According to the invention, the open steam circuit includes compressionmeans able to compress and superheat the steam to a higher temperaturethan the one of the heating fluid of the working table at continuousrunning, and at least one condenser having as refrigerating means thesaid heating fluid circulating in the first circuit in order to recoverat least part of the energy supplied to the compression means.Specifically, the compression means consists in a blower 19 having ahigher capacity than the one of the vacuum pump 16 and equipped with asafety by-pass 20 calibrated to the difference of the desired pressureto avoid overloads to its electric or endothermic entrainment motor.

The condenser 21 is placed downstream from the blower and before thevacuum pump 16, and it is equipped with a liquid separator and adischarge device 22 of a known type. The condenser has inside a coolingcoil 23, which uses as refrigerating means the heating fluid of theworking tables 2 circulating in the closed circuit.

The steam released from the products in the drying chamber 4 iscompressed by the blower 19 up to the differential pressure included,for example, between 100 and 300 mbar, preferably included betweenapprox. 100/200 mbar according to the applied power.

Due to the compression, the steam is superheated reducing its specificvolume and reaching an equilibrium temperature higher than the one ofthe heating fluid of the closed circuit of the working tables and namelyof the evaporation temperature in the drying chamber. Such difference oftemperature could be for example between 10 and 30° C. and it ispreferably included between 15 and 25° C.

The condensation heat will be therefore recovered and also thesuperheated heat generated from the mechanical energy applied to theblower. This energy will be given, in the condenser, to the heatingfluid of the tables, apart from the inevitable heat losses along theline between the blower and the condenser.

To maintain the steam temperature at the blower discharge within thelimits technologically compatible with its characteristics, thesuperheated steam from the blower must not exceed a pre-determinedmaximum value, for example next to approx. 140° C. The limit of themaximum temperature can be carried out by a thermostatic probe 24 whichcontrols a water injection device 25 upstream from the blower.

The water to be injected can be the condensation itself produced by theplant which can be sucked by the plant itself 14 by difference ofpressure with respect to the discharge of the condensation.

The steam compressed and superheated in controlled conditions isconveyed in condenser 21 where it gives off the heat of condensation andthe cooling heat to the water of the heating circuit which is surely ata lower temperature than the temperature of the equilibrium curve at thepressure conditions determined by the compression. The condensate isthen separated and discharged in the device 22.

The heating fluid, which traverses the coil of the condenser 21 fromwhich it receives the cooling heat and the condensation heat, isconveyed towards the working table 2 by means of circulator 26.

In the closed circuit there will be necessarily an expansion tank 27generally, but not necessarily, open.

Before reaching the working tables, the fluid goes through a heatexchanger 28 which has the function of absorbing and dissipating part ofthe stored heat in case it is necessary to limit the maximum temperatureof the fluid.

Also, the fluid goes through an auxiliary heat exchanger 29, which hasthe function to heat the fluid up to the continuous running temperatureand eventually to control the process. Once the evaporation process tothe desired temperature has triggered off, no thermal contribution willbe necessary from this exchanger.

From the above description, it is clear that the plant for vacuum dryersand the drier of the invention achieve all the pre-determined targets,and their simple manufacturing is specifically highlighted, because thecomplex condensers inside the working tables are avoided as well as thechilling plant and the holding of the chilled water typical of the pastand also their low initial and maintenance cost.

What is claimed is:
 1. Vacuum drier with a thermal-compression plantwith heat recovery, comprising:one or more heated working tables onwhich products to be dried are layable and each having an hermetic coverable to upwardly close said table for defining a drying chamber foraccommodating the products in said drying chamber; a heating fluidcirculating in a first closed circuit arranged for heating said dryingchamber such that the products arranged in said drying chamber areheated and generate steam from said products in said drying chamber forremoval from said drying chamber for the drying products; a second opencircuit connected to said drying chamber for the suction and removal ofthe generated steam from said drying chamber and for the condensation ofthe steam removed from the drying chamber, said second circuit includinga vacuum pump arranged proximate to an open end of said second circuitand a condenser disposed upstream of said vacuum pump with respect to aflow of the steam removed from said drying chamber by said secondcircuit; compression means disposed in said second circuit between saiddrying chamber and said vacuum pump, said compression means beingadapted to compress and superheat the steam in said second circuit to atemperature higher than that of the heating fluid for said dryingchamber at continuous running, said condenser being arranged betweensaid compression means and said vacuum pump and configured to operate ata pressure lower than the atmospheric pressure of the vacuum drier, andsaid first circuit being connected to said condenser such that theheating fluid circulating in said first circuit receives condensationheat from the steam in said second circuit in order to recover thecondensation heat for the steam in said second circuit as well as mostof the energy supplied to said compression means under the form ofsuperheated steam.
 2. The combination of claim 1, in which said vacuumpump has a relatively low capacity.
 3. The combination of claim 2, inwhich said compression means include at least a blower of a highercapacity from that of the vacuum pump and placed upstream from thecondenser in order to compress and superheat the steam before enteringthe condenser.
 4. The combination of claim 3, in which each dryingchamber includes at least a steam manifold connected to a separator ofsolid bodies through an eventual intercepting valve.
 5. The combinationof 4, in which the differential pressure of the steam between upstreamand downstream of the said blower is kept at a value generally between50 and 300 mbar.
 6. The combination of claim 5, in which saiddifferential pressure is adjusted in a way to determine an increase ofthe steam temperature with respect to the one in each drying chambergenerally included between 10 and 30° C.
 7. The combination of claim 6,in which the maximum temperature of the superheated steam downstreamfrom said blower is kept under a pre-determined maximum value by meansof an eventual water injection device.
 8. The combination of claim 7, inwhich the said maximum value of the temperature is equal to approx. 140°C.
 9. The combination of claim 8, in which the temperature at continuousrunning of the said working table is approximately included between 30and 80° C.
 10. The combination of claim 9, in which said first closedcircuit includes a circulator, a first heat exchanger to absorb anddissipate the heat with control functions of the maximum temperature ofthe heating fluid, and at least an auxiliary heat exchanger placeddownstream from the condenser traversed zone for the initial heating ofthe fluid up to the temperature at continuous running.
 11. Thecombination of claim 5, wherein the differential pressure of the steambetween upstream and downstream of the said blower is kept between 100and 200 mbar.
 12. The combination of claim 11, wherein said differentialpressure is adjusted in a way to determine an increase of the steamtemperature with respect to the one in each drying chamber generallybetween 15 and 25° C.
 13. The combination of claim 9, wherein thetemperature at continuous running of the said working table isapproximately 50° C.
 14. The combination of claim 1, wherein said firstclosed circuit includes a cooling coil of said condenser such that theheating fluid for said drying chamber flows in said cooling coil of saidcondenser.
 15. The combination of claim 14, further comprising a waterinjection device arranged in said second circuit upstream of saidcompression means.
 16. The combination of claim 15, further comprising aliquid separator and discharge device for said condenser, said liquidseparator and discharge device being connected to said water injectiondevice.
 17. The combination of claim 16, further comprising athermostatic probe connected to said compression means and said waterinjection device for controlling the steam temperature at the dischargeof said compression means.